A scanning probe microscope (SPM) has been commonly known as a device for the surface observation, surface roughness measurement or other operations on a piece of metal, semiconductor, ceramic, synthetic resin or various other materials. A representative of the SPM is an atomic force microscope (ATM) which measures an interatomic force that acts between a probe and a sample surface.
In recent years, the so-called “contact mode” and “dynamic mode” have been popularly used as the measurement techniques used in the atomic force microscope. In the dynamic mode, a cantilever provided with a probe is vibrated at or near its resonance point, and the interaction between the probe and the sample surface during the vibration is detected through a change in the amplitude, phase or frequency of the vibration of the cantilever.
FIG. 6 shows a configuration of the main components of a common type of scanning probe microscope. A sample 1 to be observed is fixed on a sample stage 2 placed on a tubular scanner 3. The scanner 3 includes: an XY-scanner 30b for driving the sample 1 in the two axial directions of X and Y which are orthogonal to each other; and a Z-scanner 30a for finely changing the position of the sample 1 in the direction of the Z axis which is orthogonal to both the X and Y axes. For each of these scanners, a piezoelectric element which produces a displacement by an externally applied voltage is provided as the drive source. A cantilever 4, with a probe 5 at its tip, is located above the sample 1. The cantilever 4 is vibrated in the vertical direction (Z-axis direction) by an excitation unit (not shown) including a piezoelectric element.
For the detection of the displacement of the cantilever 4 in the Z direction, a displacement detection unit (optical displacement detection unit) 6 including a laser source 61, photodetector 62, half mirror 63, and mirror 64 is provided above the cantilever 4. In the optical displacement detection unit 6, laser light emitted from the laser source 61 is reflected by the half mirror 63 to a substantially perpendicular direction. The reflected light falls onto a reflective surface 40 provided at the back of the tip portion of the cantilever 4. The light reflected by the reflective surface 40 of the cantilever 4 falls onto the photo detector 62 via the mirror 64. For example, the photodetector 62 is provided with a four-segment photodetector having a light-receiving surface divided into four segments arrayed in the Z-axis and Y-axis directions. If the cantilever 4 is displaced in the Z-axis direction, a change occurs in the proportions of the amounts of light incident on those four light-receiving segments. The amount of displacement of the cantilever 4 can be calculated by processing detection signals corresponding to the amounts of light received by the individual light-receiving segments.
A brief description of the measurement operation in the dynamic mode in the scanning probe microscope having the previously described configuration is as follows: The cantilever 4 is vibrated in the Z-axis direction at or near its resonance frequency by the excitation unit (not shown). If an attractive or repulsive force occurs between the probe 5 and the surface of the sample 1 during the vibration, the amplitude of the vibration of the cantilever 4 changes. The slight change in the vibration amplitude is detected through the detection signal produced by the photodetector 62, while a feedback control of the piezoelectric element in the Z-scanner 30a is performed for changing the position of the sample 1 in the Z-axis direction so as to cancel the amount of change in the vibration amplitude, i.e. to maintain the same vibration amplitude. While such a control is continuously perfoiined, the piezoelectric element in the XY-scanner 30b is controlled to change the position of the sample 1 in the X-Y plane and thereby scan the surface of the sample 1 with the probe 5. During this scan, the amount of the feedback control in the Z-axis direction mentioned earlier reflects the unevenness on the surface of the sample 1. Using a signal indicative of the amount of feedback control, a data-processing unit (not shown) creates a surface elevation image of the sample 1.
In such a scanning probe microscope, the positions of the laser source 61 and the photodetector 62 are adjusted so that the incident position of the laser light (the position at which the amount of incident light is highest) reflected by the reflective surface 40 of the cantilever 4 will coincide with the center of the four-segment light-receiving surface of the photodetector 62 under the condition that there is no deflection of the cantilever 4. Such an adjustment in the scanning probe microscope is called the “optical axis adjustment”.
For example, a conventional and typical procedure of the optical axis adjustment is as follows: Initially, an image which shows an area around the tip portion of the cantilever 4 viewed from above is taken with a video camera 8 capable of optical microscopic observation. The image is displayed on the screen of a display unit 81. Visually checking this image, an operator adjusts the position of the laser source 61 so that the spot image of laser light on the image comes to an appropriate position on the reflective surface 40 at the tip of the cantilever 4. Specifically, the operator manually adjusts the position of the laser source 61 in the Y and Z directions by operating a handle 7910 of a gear drive mechanism (first gear-drive mechanism) 791, which is provided for the laser source 61 to change its position in each of the two axial directions (Y and Z directions) which are orthogonal to each other in a plane which is perpendicular to the optical axis of the laser source 61. After the position of the laser source 61 has been fixed, the operator adjusts the position (Y and Z positions) of the photodetector 62 so that the spot of the laser light reflected by the reflective surface 40 comes to the center of the four-segment light-receiving surface of the photodetector 62. As in the case of the positional adjustment of the laser source 61, the operator manually adjusts the position of the photodetector 62 by operating a handle 7920 of another gear drive mechanism (second gear-drive mechanism) 792 provided for the photodetector 62.
However, in the previously described mode of the optical adjustment which requires manual operations by an operator, the adjustment accuracy is significantly affected by the skill level of the operator. Furthermore, a considerable amount of time is needed for the adjustment.
To address this problem, a technique for automatically performing the optical axis adjustment has been proposed in Patent Literature 1. In this technique, a controller determines the positional relationship of the cantilever and the light beam based on an optical image (i.e. an optical image including the cantilever and the light beam (actually, the laser beam spot)) taken with a CCD camera, and drives the laser source to such a position (target position) as to make the light beam from the laser light fall onto an appropriate position on the tip portion of the cantilever. Specifically, two stepping motors provided for the laser source (i.e. a stepping motor for driving the laser source along the X axis, and a stepping motor for driving the laser source along the Y axis) are individually controlled to drive the laser source to the target position.